Role Of Cyclooxygenase Products Research Articles

Adrenaline inhibits adenosine diphosphate induced intravascular aggregation of rat platelets through stimulation of 2 adrenoceptors

The aim was to determine the receptors involved in the antiaggregatory effects of adrenaline in the rat. The 111indium oxine technique was used to evaluate ADP induced pulmonary accumulation of 111In labelled platelets in adrenalectomised rats (ie, no endogenous adrenaline) in the absence of anticoagulants. Adrenaline was infused (10, 20 and 40 micrograms.kg-1.h-1) and ADP aggregation measured. Adrenoceptor agonists, isoprenaline, methoxamine, and BHT 933, were given to evaluate whether they mimic the adrenaline effects. Adrenoceptor antagonists, yohimbine, propranolol, prazosin, and WY 26392, were given to assess whether they block the adrenaline effects. In all cases, pulmonary accumulation of platelets was measured in response to ADP. Adrenaline dose dependently inhibited platelet aggregation. BHT 933 (1.5 micrograms.kg-1.min-1) mimicked, while phentolamine (1 mg.kg-1), yohimbine (2.5 mg.kg-1), and WY 26392 (0.1, 1.0, 5.0 mg.kg-1) blocked, the adrenaline effects. Methoxamine (3.3 micrograms.kg-1.min-1) produced a relatively weak non-specific inhibition of ADP aggregation. Isoprenaline (33 ng.kg-1.min-1) did not mimic the effects of adrenaline, neither did propranolol (1 mg.kg-1) affect the adrenaline inhibition of the aggregatory responses to ADP. Indomethacin (3 mg.kg-1) blocked the inhibitory effects of adrenaline. Adrenaline stimulates alpha receptors on platelets but inhibits platelet aggregation. There is no evidence to suggest that beta receptors participated in the effects produced by adrenaline. There is a role for cyclo-oxygenase products in the inhibitory effects of adrenaline on aggregation.

Contractile effect of the chemotactic factors f-Met-Leu-Phe and C5a on the human isolated umbilical artery. Role of cyclooxygenase products and tissue macrophages.

Factors that are chemotactic for phagocytic leukocytes are known to elicit important acute circulatory changes, and the role of circulating leukocytes in these models is controversial. To evaluate the role of the blood vessel wall in the absence of circulating cells, spiral strips of human umbilical artery were exposed in vitro to the chemotactic peptides f-Met-Leu-Phe (1-100 nM) or C5a (2.5-25 nM). Contractile responses were observed for both peptides. Certain agonist analogues and a selective antagonist of the chemotactic action of f-Met-Leu-Phe behaved correspondingly as agonists and antagonist of the contractile effect on umbilical artery. The anaphylatoxin C3a also exerted a contractile effect on the tissue (25 nM and above), but this effect was highly tachyphylactic. Inhibitory drugs were used to examine the contributions of secondary mediators in eliciting the effects of C5a and f-Met-Leu-Phe. The contractile effect of both peptides was massively inhibited either by indomethacin or the thromboxane A2/prostaglandin H2 antagonist SQ 29548. Dazmegrel, a thromboxane A2 synthetase inhibitor, had partial inhibitory effects on contractions induced by either peptide. The contractile effect of C3a was prevented by indomethacin pretreatment. Vascular strips did not release measurable histamine in the bathing fluid after challenge with C5a or f-Met-Leu-Phe. The tissue apparently contains neither histamine nor mast cells. Autoradiography of 125I-labeled C5a or f-Met-Leu-Phe analogue showed specific binding of the peptides to cells dispersed in the vessel wall, but more frequently at the periphery. Cells stained positively for alpha-naphthyl acetate esterase showed a similar distribution. Pure cultures of smooth muscle cells derived from the umbilical artery failed to release prostanoids when exposed to f-Met-Leu-Phe or C5a, whereas fresh strips of this artery released more thromboxane B2 than the baseline in response to these peptides. We conclude that macrophagelike cells, present in the vessel wall, are the likely target cells for the chemotactic peptides. These cells trigger a contractile effect of the smooth muscle by generating cyclooxygenase products.

Reversal of Tachyphylaxis to Angiotensin II in Dog Lung

Angiotensin II (ANG II) is a potent vasoconstrictor in most vascular beds. We studied the role of cyclooxygenase products and/or endothelium-derived relaxing factor (EDRF) in modulating the pressor response to ANG II in the isolated, blood perfused dog lung. ANG II was given as a bolus dose of 2, 4 and 8 micrograms before and after cyclooxygenase inhibition (COI) with either 40 mumol/l indometacin (INDO) or 45 mumol/l meclofenamate (MECLO), and before and after methylene blue (MB) infusion followed by MECLO or MECLO followed by MB infusion. ANG II produced an increase in lobar vascular resistance (LVR) that averaged 3.7 +/- 1.1 to 3.0 +/- 0.3 cm H2O/l/min (n = 30), but was not dose-related and exhibited marked tachyphylaxis. In contrast, after INDO, the increase in LVR to ANG II averaged 8.2 +/- 1.0 to 18.4 +/- 2.2 (n = 6) and 5.0 +/- 1.2 to 15 +/- 2.4 cm H2O/l/min after MECLO (n = 6) and both cyclooxygenase inhibitors increased (p less than 0.05) basal vascular tone. Infusion of MB did not alter baseline vascular tone, but prevented the tachyphylaxis to ANG II. Our results indicate that tachyphylaxis to ANG II-induced vasconstriction in the isolated, blood perfused dog lung lobe is not only reversed by COI, but potentiated and dose-related. Whereas MB diminished tachyphylaxis to ANG II, it failed to potentiate the pressor response to ANG II except with concurrent COI. Our findings suggest that vasodilator cyclooxygenase products are probably more important than EDRF in regulating both vascular tone and reactivity to ANG II in the dog lung.

Effect of indomethacin on allergen-induced asthmatic responses

Previous studies have suggested that inhibition of the cyclooxygenase pathway of arachidonic acid metabolism may suppress the late asthmatic responses to inhaled allergen. Both human and animal studies have suggested that prostanoids may also be involved in increases in airway responsiveness after ozone and allergen. We studied seven atopic subjects, who had a dual asthmatic response to inhaled allergen, during a control period and then after pretreatment with indomethacin (50 mg) or placebo twice daily for 2 days, administered in a randomized, double-blind manner. Indomethacin had no significant effect on the base-line airway responsiveness to histamine (P = 0.22) or the allergen-induced early or late asthmatic response (P = 0.49). However, indomethacin inhibited the increase in airway responsiveness (express as histamine PC20) after allergen inhalation. The log difference in preallergen to postallergen histamine PC20 was 0.49 +/- 0.08 (SE) during the control period, 0.46 +/- 0.08 (SE) after placebo (P = 0.81), and 0.22 +/- 0.10 (SE) after indomethacin (P = 0.02). Although indomethacin is useful for examining the role of cyclooxygenase products in asthmatic responses, it should not be considered in the treatment of asthma. We conclude that cyclooxygenase products are not significant mediators of allergen-induced early or late asthmatic responses but are involved in the pathogenesis of airway hyperresponsiveness after allergen inhalation.

Can the Effects of Protein Restriction on the Progression of Renal Failure Also Be Achieved by Pharmacological Means?

A review is given of recent data on the role of cyclo-oxygenase products in mediating glomerular hyperfiltration in renal disease and on the role of prostaglandins in protein-induced glomerular hyperfiltration. The possible beneficial effects of cyclo-oxygenase inhibitors and thromboxane synthetase inhibitors are discussed.

Meclofenamate blocks the pulmonary arterial vasopressor effects of leukotriene B 4 in awake sheep

This study examined the hemodynamic effects of leukotriene B 4 (LTB 4 in chronically instrumented awake sheep, and the role of cyclooxygenase products in the sheep's response to LTB 4. LTB 4 (25 μg) was given as a into the pulmonary artery. Six sheep were studied with LTB 4, both before and after pre-treatment with meclofenamate (5mg/kg load, 3 mg/kg/hr maintenace infusion). LTB 4 alone caused a rapid rise in pulmonary arterial pressure from 15 ± 1 to 42 ± 11 cm H 2O. LTB 4 had no effect on pulmonary arterial pressure following pre-treatment with meclofenamate. LTB 4 alone caused an increase in serum thromboxane B 2 (TxB 2) from 130 ± 35 to 320 ± 17 pg/ml 3 minutes after dosing but did not increase TxB 2 following pre-treatment with meclofenamate. LTB 4 caused a slight decrease in mean systemic arterial pressure and a transient fall in circulating white bloos cells, both of which were unaffected by meclofenamate pre-treatment. The vasoactive effects of LTB 4 in the pulmonary circulation appear to be mediated indirectly through the production of cyclooxygenase metabolites of arachidonic acid.

Postasphyxial increases in prostanoids in cerebrospinal fluid of piglets.

The dilator stimuli that contribute to postasphyxial increases in cerebral blood flow in the neonate are unclear. To assess the possible role of cyclooxygenase products in these responses, we measured pial arteriolar diameter in six piglets and determined levels of prostaglandin (PG) E2 and 6-keto-PG F1 alpha (hydrolysis product of PGI2) in cerebrospinal fluid (CSF) bathing the parietal cortex during control conditions, after 4-10 min of complete respiratory arrest (asphyxia), and after 5-12 min of reventilation. Pial arterioles are important resistance vessels in the cerebral circulation. Baseline pial arteriolar diameter was 220 +/- 40 micron (mean +/- SEM) and increased to a maximum of 252 +/- 49 and 267 +/- 56 micron after asphyxia and reventilation, respectively. During control conditions, CSF PGE2 (n = 6) and 6-keto-PGF1 alpha (n = 4) levels were 1947 +/- 310 and 794 +/- 147 pg/ml, respectively. During asphyxia, CSF levels of PGE2 did not increase, whereas 6-keto-PGF1 alpha increased modestly. During reventilation, CSF PGE2 increased to 3576 +/- 499 pg/ml, and 6-keto-PGF1 alpha increased to 2846 +/- 123 pg/ml. In other experiments, we determined that these CSF levels of PGE2 and PGI2 (as 6-keto-PGF1 alpha) were within the vasodilator range for pial arterioles. We conclude that postasphyxial increases in pial arteriolar diameter are associated with a rise in CSF levels of dilator prostanoids.

Bronchial circulation and cyclooxygenase products in acute lung injury.

The role of cyclooxygenase products in the response of the bronchial circulation to acute lung injury was examined in 30 dogs. By use of an open-chest preparation the left lower lobe (LLL) pulmonary circulation was isolated, continuously weighed, and perfused in situ. The anastomotic bronchial blood flow [Qbr(s-p)] was measured as the rate of increase in the volume of the LLL-perfusion circuit. Four groups of dogs were studied. In group A, six dogs received cyclooxygenase inhibition (COI) with either indomethacin (2 mg/kg) or ibuprofen (10 mg/kg). In group B (n = 10) lung injury caused by airway instillation of glucose (15 mg) with glucose oxidase (500 micrograms/kg) (G/GO) or LLL pulmonary arterial infusion of alpha-napthyl thiourea (ANTU, 2 mg/kg). Group C (n = 10) received COI, and 30 min later injury was induced as above with either ANTU or G/GO. Group D (n = 4) received COI immediately after anesthesia; then, 30 min after completion of the surgical preparation, injury was induced with ANTU or G/GO. After COI, Qbr(s-p) decreased to 35 +/- 9% of the basal values (P less than 0.05). After administration of ANTU or G/GO, Qbr(s-p) increased irrespective of whether COI was present. 6-Ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TxB2) were measured by radioimmunoassay in the LLL pulmonary artery and systemic venous blood, demonstrating an increase in 6-keto-PGF1 alpha due to surgical preparation and confirming complete COI in those animals receiving COI immediately after anesthesia. These findings demonstrate that 1) the bronchial circulation is capable of a sevenfold increase in flow in response to acute lung injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Endotoxin-induced alterations in guinea pig coronary vascular beta 2-adrenoceptor function: a role for cyclo-oxygenase products.

Bordetella pertussis and in particular its cell wall constituent endotoxin induces a hyporesponsiveness of the coronary vascular beta2-adrenoceptor and a hyperreactivity of the alpha2-adrenoceptor in the guinea pig coronary vascular system [5]. In literature, much attention has been paid to betaadrenoceptor desensitization and several hypotheses have been proposed in order to explain the molecular mechanisms of this phenomenon [4]. The involvement of mediators other than catecholamines has been considered. Among these mediators, arachidonic acid and its metabolites have been suggested to play a role in desensitization of beta-adrenoceptors in various experimental designs [2, 3, 4]. Since endotoxin is a powerful stimulator of the immune system and thus initiates the biosynthesis of various inflammatory mediators among which are the arachidonic acid metabolites, it seems of particular interest to examine the hypothesis that arachidonic acid metabolites are involved in beta2-adrenoceptor impairment in the coronary vascular system of the guinea pig heart after endotoxin. Furthermore, these experiments might also elucidate the possible role of prostaglandin synthesis for alpha2-adrenoceptor hyperreactivity.

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31. Piper PJ, Samhoun MN: Stimulation of arachidonic acid metabolism and generation of thromboxane A? by leukotrienes B,, C,, ant! D, in guinea pig lung in vitro. Br J Pharmacol 77:267, 1982 32. Sirois P, Chagnon M, Borgeat P, Vallerand P: Role of cyclooxygenase products in the lung action of leukotrienes A,, B,, C,, D,, and E4. Pharmacology (in press) 33. Robinson c’, Hoult JRS, Waddell KA, Blair IA, Dollery CT: Total profilmg by GCiNICIMS of the major cyclooxygenase products from antigen and leukotriene-challenged guinea pig lung. Bioclem Pharmacol 33:395, 1984 34. Piper PJ, Scale JP: Nonimmunological release of slow-reacting substance from guinea pig lungs. Br J Pharmacol 67:67, 1979 35. Borgeat P, Samuelsson B: Metabolism of arachidonic acid in poiymorphonuclear leukocytes: effects of the ionophore A23187. Proc Natl Acad Sci USA 76:2148. 1979 36. Jubiz W, Radmark 0, Malmsten C. Hansson G, Lindgren JA. Palmblad J, UdCn AM, Samuelsson B: A novel leukotriene produced by stimulation of leukocytes with formyhnethionylleucylphenylalanine. J Biol Chem 257:6106. 1982

A role for cyclooxygenase products in the formation of phosphatidic acid in stimulated human platelets. Differential mechanisms of action of thrombin and collagen.

Human platelets prelabeled with (32P)orthophosphate or [14C]arachidonic acid (AA) were stimulated with collagen or thrombin, and platelet activation (shape change, aggregation, and release of serotonin) was determined in parallel to the formation of 32P- or 14C-labeled phosphatidic acid (PA). The results show a close correlation between the degree of platelet activation and the amount of PA formed. Activation of platelets and formation of PA induced by collagen (2 to 20 micrograms/ml) was blocked by pretreatment of platelets with trifluoperazine, indomethacin, aspirin, or N-methylimidazole. This suggests that the formation of AA by phospholipase A2 and its subsequent metabolism by cyclooxygenase and thromboxane synthetase are required for the collagen-induced formation of PA. Endoperoxide analog U-44069 induces formation of PA in human platelets that have been pretreated with or without aspirin. The action of thrombin does not follow the same pattern of collagen. Low concentrations of thrombin (0.05 units/ml) induce only platelet shape change and a small stimulation of PA, changes which are only minimally inhibited by indomethacin. However, a small increase in the thrombin concentration (to 0.1 unit/ml) induces platelet aggregation, release of serotonin, and a sharp increase in PA accumulation which are effectively inhibited by indomethacin. Even higher thrombin concentrations (0.4 to 0.8 units/ml), however, result in a further stimulation of PA formation, platelet aggregation, and release of serotonin which are insensitive to inhibition by indomethacin. The data show that cyclooxygenase metabolites of AA, produced after platelet activation, may be differentially involved in the formation of PA in platelets stimulated with collagen or thrombin. Formation of PA following collagen or intermediate concentrations of thrombin (0.1 to 0.2 units/ml) is dependent on the cyclooxygenase pathway. However, formation of PA by very low or by high concentrations of thrombin is not mediated by cyclooxygenase metabolites of AA.